The present invention is directed to a method for manufacturing of light waveguide couplers having three or more gates and utilizing a beam splitter principle and to the waveguide couplers produced by the method.
A method of forming a light waveguide coupler that uses a beam splitter principle is disclosed in U.S. Pat. No. 4,317,699, whose disclosure is incorporated by reference thereto and which patent claimed priority from the German Application No. 28 51 679. As disclosed, a carrier member supports a branching light waveguide structure and is manufactured with a branching guide channel structure comprising a through guide channel from which guide channels branch off. This channel structure is first produced in a surface of a carrier member, then a through glass fiber light waveguide is placed in the through guide channel and a glass branching fiber light waveguide is placed in each of the branching guide channels and is arranged with the branching fiber abutting laterally against the fiber in the through channel. To permanently fix the fibers in the guide channels, the branching light waveguide structures is covered with a cover member which is permanently connected to the carrier. Subsequently, the member with the cover secured thereon is cut into two parts, the cut surfaces formed in each of the parts are then polished to an optical quality and then a coating step occurs to provide a beam splitting layer and the two parts are then rejoined together with the waveguides of one part being aligned with the waveguides of the other part.
In the step of manufacturing the branching guide channel structure, the surface of the main carrier is covered with a mask having spaces corresponding to the guide channel structure and this structure is subsequently etched into the carrier member. The method of the patent also discloses providing polishing marks which will indicate when the polishing of the cut surfaces should end so that the beam divider provided on the polished cut surface is in the desired position relative to the waveguide structure when the parts are reassembled to form the coupler.
Another type of branching light waveguide structure is manufactured by an ion exchange method in a carrier member of glass. Such a method is disclosed in an article by Eiji Okuda et al "Planar gradient-index glass waveguide and its applications to a 4-port branched circuit and star coupler", Applied Optics, Vol. 23, No. 11, June 1, 1984, pages 1745-1748. As disclosed in this article, a continuous waveguide structure is formed in a substrate or member of optically homogeneous glass by providing a metal film, such as of titanium, on a surface of the glass substrate with this metal film acting as a mask against the ion diffusion. The desired pattern for the branch waveguide is formed by selective etching of this film and preferrably, the glass substrate is an alkyli borosilicate glass. The glass substrate is then immersed in a molten salt bath containing A-ions that will increase the refractive index of the glass with the bath being at a temperature of 500.degree.-600.degree. C. The A-ions will diffuse through the exposed portions of the mask into the glass substrate and this diffusion is accelerated by applying an electrical field in the direction of the thickness of the glass member. This will result in a high refractive index pattern with a half circle cross section being formed beneath the glass surface. After this first step, the mask pattern is removed by etching and the substrate with the higher index-refraction pattern is immersed in a second bath which is at 500.degree.-600.degree., of molten salts containing B-ions that will modify the refractive index distribution in the glass substrate. These B-ions are diffused in the glass substrate in the same way as the first stage so that a buried waveguide structure will be produced. It is noted, that this article discloses forming branch circuits and star couplers but does not suggest forming couplers utilizing beam splitter principle.
A five gate wavelength selective branch coupler, which acts or utilizes a beam splitter technique, is disclosed in U.S. Pat. No. 4,564,755 whose disclosures is incorporated by a referenced and which claims priority from German application No. 32 30 570. In this coupler, three thick waveguides and two small diameter waveguides are arranged to form the five gate coupler. Two of the thick waveguides have a portion removed along one side to form a ground surface that receives a small diameter waveguide. One of these thick waveguides, with the small diameter waveguide, is aligned with the axis of another thick waveguide with the beam splitter extending therebetween at an angle of approximately 45.degree. to the axis of the thick waveguides. Another pair of waveguides are arranged at right angles to the single thick waveguide and includes the other thick waveguide having a ground surface which receives a small diameter waveguide. The two small diameter waveguides are arranged with their axes intercepting in the plane of the beam splitter.